1. Field of the Invention
The present invention relates generally to enhancing communication capabilities in a communication system, and in particular, to a method and apparatus for canceling acoustic echo in a communication system providing a TTY/TDD service.
2. Description of the Related Art
Communication technology, particularly mobile communication technology, has been developed to additionally provide data service as well as voice service. Communication systems provide additional services to satisfy various user demands. One of the additional services is a TTY/TDD service. The term “TTY” derives from “Teletype”, which is a registered trademark of the Teletype Corporation. “TDD”, is an abbreviation of the phrase “telecommunications device for the deaf”. TTY/TDD services enable telephone calls to the deaf or hearing-impaired people.
Implementation of the TTY/TDD service requires a TTY/TDD device. The TTY/TDD device comprises a keyboard, a MODEM (Modulator/DEModulator), and a display connected to a wired telephone via the MODEM. The TTY/TDD device converts PCM (Pulse Code Modulation) signal from the other party to text messages and displays them on the display. It also converts text messages received from the keyboard to PCM signals and provides the PCM signal to the wired telephone. Thus, the deaf can conduct a telephone call.
In connection with the TTY/TDD service, the U.S. has created ground rules to provide services to the deaf and hard of hearing persons since the late 1970's. In 1990, the ADA (the Americans with Disabilities Act) was instituted to enforce the rights of individuals with disabilities. Today, it has been decided that a TTY/TDD functionality shall be provided in the standards of a mobile communication system. As is well known to those skilled in the art of the invention, 13K QCELP (Qualcomm Code Excited Linear Prediction Code) and EVRC (Enhanced Variable Rate Code) vocoders are widespread voice compression techniques in 2nd and 3rd generation wireless communications systems. The TTY/TDD functionality is implemented as IS733-3 and IS127-4 in these vocoders. IS-733-3 defines the TTY/TDD functionality of the 13K QCELP vocoder, and IS-127-4, that of the EVRC vocoder.
The encoder of such a vocoder processes voice samples on a 20-ms frame basis and transmits processed voice information in packets. The decoder of the vocoder reproduces the voice samples by processing received voice packets in an order reverse to the encoder operation. When the TTY/TDD functionality is implemented, the encoder determines whether input PCM (pulse code modulation) samples are voice or TTY/TDD text. In the case of voice, the encoder transmits voice information in packets. In the case of TTY/TDD text, it transmits text information in packets. The decoder determines whether a received packet is voice information or TTY/TDD text information and reproduces voice samples or TTY text tones according to the result of the determination. The TTY/TDD functionality supports voice carryover/hearing carryover (VCO/HCO). VCO allows people who have difficulties in hearing clearly through the telephone to use a TTY without typing. They only use the TTY for reading during the inbound direction of the call. They talk to their party by speaking into a microphone. Similarly, HCO allows a person with a speech disability to hear a response from their party directly. A communication system equipped with vocoders having these options enables communications between a person without any speech or hearing disabilities and a hearing-impaired person or between hearing-impaired persons.
Communication systems equipped with vocoders having the TTY/TDD functionality are illustrated in FIGS. 1, 2A and 2B. FIG. 1 illustrates the network configuration of a communication system that allows a TTY/TDD service between a mobile subscriber and a subscriber to the public switched telephone network (PSTN), and FIGS. 2A and 2B illustrate the network configuration of a communication system that allows a TTY/TDD service between mobile subscribers.
The TTY/TDD service in these communication systems has at least the following two problems:
(1) In the case when a mobile-to-land call (an MtoL call) is established between a mobile and PSTN subscriber (voice transmission/reception), as illustrated in FIG. 1, the acoustic echo that can be generated from a land phone 111 will not negatively impact the communication quality of the call as perceived by the mobile phone (TTY/TDD phone) 103. The reason why the call-quality is not negatively impacted is because the signal length and delay of the acoustic echo are inaudibly small. That is, under an environment with voice data only, even if acoustic echo is caused in the land phone 111, the strength of a PCM signal delivered from the speaker to the microphone of the land phone 111 is very small and the microphone is sufficiently apart from the speaker. Thus the communication quality the TTY/TDD phone 103 perceives is good. Therefore, the acoustic echo is not a problem.
However, in the case of transmission/reception of TTY/TDD text information, a TTY/TDD device 101 substantially perceives the signal strength of the acoustic echo generated from the land phone 111. The TTY/TDD device 101 then reproduces the TTY/TDD information due to the acoustic echo involved with transmission of the TTY/TDD information. Specifically, under an environment having coexistent voice and TTY/TDD text, a PCM signal of TTY/TDD text tones output from the speaker of the land phone 111 is delivered to its microphone. However small its signal strength is, an encoder 105b in a vocoder 105 within a base station (BS) 105 perceives the PCM signal substantially and transmits the TTY/TDD text information in a packet to the TTY/TDD phone 103. Therefore, the TTY/TDD device 101 connected to the TTY/TDD phone 103 reproduces the TTY/TDD text. Consequently, the acoustic echo generated from the land phone 111 influences the TTY/TDD device 101.
(2) When a mobile-to-mobile (MtoM) trans-coding call is established between mobile subscribers as illustrated in FIGS. 2A and 2B, acoustic echo caused in one TTY/TDD phone 311 leads to reproduction of transmitted TTY/TDD text information in a TTY/TDD device 301 connected to the other TTY/TDD phone 303. Under the environment of voice data only, even if acoustic echo is generated from the TTY/TDD phone 311, the strength of a PCM signal delivered from its speaker to its microphone is negligibly small. Therefore, there is no influence on the communication quality of the other TTY/TDD phone 303.
In the environment of voice information coexisting with TTY/TDD text information, however, PCM TTY/TDD text tones delivered from the speaker to the microphone of the TTY/TDD phone 311 are substantially sensed in its encoder 311b. The encoder 311b will sense the PCM TTY/TDD text tones because the encoder 311b is specifically configured to sense the TTY/TTD text tones, however small they are, in accordance with the “3GPP2 C.S0028 Version 1.0 CDMA TTY/TDD Minimum Performance Specification.” Therefore, the TTY/TDD device 301 connected to the other TTY/TDD phone 303 reproduces the TTY/TDD text due to the acoustic echo.